Variable capacitances are often used in pressure sensors, for example. In such applications, it is required to determine the value of the variable capacitance. The pressure applied to the sensor changes the value of the variable capacitance, which allows for measurement of the pressure. A simplified circuit diagram of a conventional circuit for measuring a variable capacitor value is shown in FIG. 1. The variable capacitor CX is arranged as an input capacitor and coupled to a negative input of an operational amplifier 102. A reference capacitor CREF is used as a feedback capacitor coupled between an output and a negative input of the operational amplifier 102. The output voltage VOUT of the operational amplifier 102 is a function of the ratio CX/CREF. The input voltage VIN applied to the variable input capacitor CX has a rectangular waveform (i.e., it is typically a square wave signal). The output voltage VOUT is also square wave. VOUT is converted into a digital signal DOUT through an analog-to-digital converter or ADC 104. The amplitude of the digital signal DOUT is proportional to the capacitance of capacitor CX. The absolute values of capacitors CREF and CX and many other parameters of the circuit have an influence on the amplitude of the output signal VOUT and therefore on the amplitude of digital signal DOUT. Other prior art solutions apply a square wave signal directly to the variable capacitor and convert the signal across the variable capacitor into a digital signal, which is evaluated for determining the pressure. Therefore, many prior art solutions require calibration before the system can be used. However, even after calibration, varying operating conditions can affect the measurement results, the precision, and the overall performance. Previous methods of determining the variable capacitance are therefore imprecise, complex, introduce large errors into the measurements, and cannot be performed over a wide range of values of the variable capacitances. Furthermore rectangular or square wave excitation is sensitive to parameter variations and rather limited in terms of frequency or bandwidth.